1. Field of the Invention
The present invention relates to a driver comparator circuit having a driver function of outputting a signal via a single transmission line, and a comparator function of judging the input signal level.
2. Description of the Related Art
In a case in which data is transmitted and received between two semiconductor devices, in some cases, bidirectional transmission is performed via a single transmission line. In a case in which a device including such a bidirectional interface is tested, the signal output from a device under test (DUT) is compared with a threshold voltage so as to judge the quality of the DUT.
Description will be made regarding a test apparatus which tests a DUT having a bidirectional interface. The test apparatus includes a transmitter (driver) and a receiver (comparator) connected to a common transmission line. The driver transmits a test pattern to the DUT. The comparator judges the logical value of a signal output from the DUT, or checks the amplitude of the signal.
The comparator of the test apparatus is connected to the DUT via the transmission line. Furthermore, the comparator of the test apparatus is connected to the driver on the test apparatus side. Accordingly, there is a need to design such a DUT test apparatus including a bidirectional interface so as to free its comparator from being affected by the output of the driver arranged adjacent to the comparator.
In the most primitive method, the output of the driver is set to a predetermined fixed terminal voltage during a period in which a signal output from the DUT is transmitted via the transmission line, thereby providing the bidirectional transmission. However, such a method has a problem in that, when the signal transmission direction is switched, overhead (which is also referred to as “round trip delay”) occurs according to the transmission line time length. With a test apparatus which supplies a very long test pattern to a DUT so as to judge the signal output from the DUT, such a round trip delay leads to a problem of an increased test period. This becomes a cause of reduced throughput.
Such a problem can occur not only in such a test apparatus, but also in bidirectional transmission between semiconductor devices. In this case, such round trip delay becomes a cause of a reduced transmission rate.
In order to solve such a round trip delay problem, various methods have been proposed. For example, Patent documents 1 and 2 disclose a circuit (hybrid circuit) in which, in bidirectional communication, a circuit block cancels the signal it transmits itself so as to receive only a signal output from another circuit block.